Various thermal treatment systems have been, and continue to be, used to separate volatile from non-volatile substances. For example, thermal desorption units are commonly used to remove substances such as mercury and volatile organics from solids. The use of vacuum retorts for this purpose is known.
The use of a vacuum decreases the boiling point of volatile substances and decreases the number of molecular collisions per unit of space in time. By minimizing these molecular collisions, chemical reactions and decompositions can be decreased so that vaporization and separation process can be efficiently and productively utilized.
For example, U.S. Pat. No. 5,569,154 (Navetta) discloses an indirectly heated continuous non-rotating vacuum retort with an internal rotating screw feeder. Navetta teaches to load the system at ambient temperature through a rotary air lock or triple dump valve to maintain negative system pressure. An internal hollow screw feeder is used to mix and transport the material being treated through the vessel. Hot gases are passed through the hollow screw feeder to indirectly heat the material being treated within the retort. The hot processed solids exit the opposite end of the screw feeder through a second rotary air lock or triple dump valve to again maintain the negative system pressure. The evolved retort gases diffuse and/or are swept with purge gases into the off-gas treatment system where they are condensed.
Additionally, U.S. Pat. No. 5,453,562 (Swanstrom) discloses an indirectly heated batch non-rotating vacuum retort with an internal screw mixer. Swanstrom teaches to load the retort at ambient pressure and temperature, seal the vessel and internally mix the stationary vessel contents with a screw mixer while indirectly heating the vessel and applying medium to high vacuum. Once the process is complete, the heat is turned off, the vacuum released, and the material removed with a screw feeder at ambient pressure. The evolved retort off-gases diffuse and/or are swept with purge gases into the off-gas treatment system for removal from the gas phase.
These non-rotating systems employ stationary vessels with internal moving flights or screws. Difficulty in uniformly heating the flights and stationary vessel often occurs, leading to metallurgical failures and reduced equipment life. Often, these systems are operated at a lower temperature than the rotary vessel systems to minimize thermal stresses. The rotating retort evenly distributes the indirect heat allowing the use of higher temperatures with less thermal stress. In addition, the mixing dynamics are different between the non-rotating retort and rotating retort systems. Moreover, particle size reduction is extremely important, especially in ambient and low vacuum systems. In a high vacuum environment, the large pressure gradient between the interior of the particle and the vacuum space drives the volatilization of substances within the particles thereby reducing the need for extensive particle size reduction. The reasons these systems do not employ a rotating retort to overcome these problems is because of the difficulty in maintaining a high vacuum on a hot rotating vessel. The seals on a non-rotating system are simple and straight forward gaskets.
Several prior art systems disclose either heated rotating vessels under slight vacuums of less than 1 inch of mercury or heated non-rotating vessels operated at high vacuums of over 28 inches of mercury. The slight vacuum employed in these rotating systems is to prevent leakage of environmentally-regulated substances out of the retort and off-gas treatment system, while the high vacuum in non-rotating systems serves to shorten process times. Although the technology is well known, there are several drawbacks and limitations.
First, in the prior art low vacuum rotating systems, complex off-gas treatment equipment is required to remove contaminated particulates and regulated chemicals prior to discharge of the treated gases to the atmosphere. This complex off-gas treatment equipment is very large and expensive compared to the system's processing rate. Due to ever more stringent air emission regulations and the need to protect human health and the environment, these off-gas treatment systems continue to become even more sophisticated and costly. One of the primary reasons that the off-gas processing systems associated with these prior art thermal units are so complex and expensive is because of the high volume of contaminated particulates and combustion, sweep, and/or leakage gases exhausted from the retort during operation.
To reduce the size and complexity of the off-gas treatment systems, indirectly fired retort vessels are often used. Heat is applied to the outside of the retort or applied with resistance heaters. These systems reduce the amount of particulates and eliminate the combustion gases exiting the retort. The prior art systems, however, do not entirely eliminate the carry out of particulates from the retort and still require a relatively large amount of sweep gas to move the vaporizing chemicals out of the retort. Therefore, even though an improvement, prior art indirectly fired retorts still require relatively large and expensive off-gas treatment systems.
Additionally, there are many cases in which one or more of the components of the matrix and/or the substances to be separated are thermally sensitive. That is, one or more of the substances break down to unwanted substances and/or the structure of one or more of the matrix components are altered that adversely affects subsequent treatment or reuse. Prior art systems employing heat and vacuum can be used for these situations. The use of vacuum lowers the boiling point of substances and, depending upon the substances involved, may allow the separation of volatile from non-volatile substances at below critical temperatures.
Additionally, the smaller the particle size, the greater the particle surface area, the faster the processing time, and the better the ultimate removal of the volatile species. The rotating retort is better in reducing particle size during processing and minimizing the production of clinkers compared to non-rotating systems employing internal mixing devices. Steel balls, chains, and similar devices can be added to the rotating retort to further improve particle size reduction capabilities during processing.
Moreover, U.S. Pat. No. 5,628,969 (Aulbaugh) discloses an indirectly heated batch rotary vacuum retort. Aulbaugh teaches to load the retort at rest at ambient pressure and temperature, seal the vessel and rotate the vessel to mix the contents while indirectly heating the vessel and applying medium to high vacuum. Once the process is complete, the heat is turned off, the vacuum released, and the material removed with a screw feeder at ambient pressure. The evolved retort off-gases diffuse into the off-gas treatment system for removal from the gas phase.
In addition, U.S. Pat. No. 5,517,004 (Blonk) discloses an inductively heated continuous rotary vacuum retort operating at below 3 millibar pressure. Blonk teaches to load the retort continuously from one of two vacuum chambers with dry bulk solids. When one chamber is empty, that chamber's discharge valve is closed and the full chamber's valve is opened. The retort vessel rotates to move the solids to the discharge point while heating the solids and applying a vacuum of zero pressure absolute to 3 millibar. The processed solids are continuously discharged at processing temperature into one of two evacuated chambers. When one chamber is full, that chamber's valve is closed and the empty chamber's valve is opened. The evolved retort off-gases are swept into the off-gas treatment system with carrier gases for removal from the gas phase. Blonk teaches a complicated and expensive method for loading dry bulk solids into a vacuum rotary retort and unloading hot processed solids from a rotary vacuum retort while processing at temperature under a very high vacuum. This system requires four stationary vacuum vessels, two for the load end and two for the unload end of the process, does not handle wet materials, must operate at extremely low pressures, and uses swept or purge gases to transport the volatile contaminants out of the retort.
The vacuum systems of the prior art allow or purposefully introduce air and very low boiling point inert purge gases, such as nitrogen, into their systems. Purge gases are often introduced to flush vapors out of the retort and into an off-gas treatment system. These gases, after commingling with the pollutant vapors, are introduced into treatment systems that attempt to separate the pollutant vapors from the gases. Air enters these systems when the vessels are loaded and unloaded and/or enters through the metallic rotary air lock and triple dump valves during processing. All off-gas treatment systems are designed to remove pollutants from a gas stream that will eventually be exhausted to the atmosphere. As the amount of the pollutant in the gas stream decreases, it is increasingly difficult and expensive to continue to remove it.
The vacuum in these prior art systems must be maintained by use of one or more vacuum pumps with a rated cfm capacity higher than the influx rate of the gases. After establishing a vacuum in these prior art systems, if the vacuum pump is turned off, the influx of gases and the production of vapors would soon allow the system pressure to return to ambient conditions. The presence of significant volumes of gases that ultimately pass through the off-gas treatment system acts in many ways to transport pollutants through the off-gas treatment system and dramatically increase the size and complexity of the system designed to reach ever more stringent air pollution control limits. Additionally, these gases impart a large amount of momentum to pollutant vapors and continuously push them through the treatment system as the gases rush in and through treatment system to the exhaust stack.
The consequences of the presence of significant amounts of these gases in the system are staggering. The prior art teaches mass collection for shipment to alternate location for disposal. Off-gas treatment equipment is extremely large, complicated, and costly and pollutants are still continuing to be spewed into the air at rates detrimental to human health and the environment. In addition, the prior art does not attempt to separate the volatile substances collected into different fractions to be collected and recycled. A thermal processing system is needed to overcome the vast limitations of prior art thermal systems by dramatically reducing system costs and complexity and decreasing pollutant emissions to the lowest level practically achievable.
Therefore, a simplified, far more versatile, and economical indirectly heated continuous rotary vacuum retort that minimizes off-gas treatment equipment and produces near zero retort off-vapor emissions is needed to process solids of widely varying particle size, liquid content and shape at higher temperatures and under wider vacuum conditions than currently exists. Additionally, there exists a need to recover and reuse the resources comprising these off-vapor emissions by collection and separation of the off-vapor emissions into useful and productive components so that the economic value of these otherwise wasted resources can be realized while offering a reduction in emissions to the lowest level practically achievable.